Water retention resulting from chronic heart failure can be detected by a number of methods, most commonly by measurement a weight gain or an increase in limb volume.
The WHARF study by Goldberg et al. (Am Heart J. Volume 146(4), pages 705 to 712, 2003) demonstrated that compliant patients monitoring their weight on a regular basis could reduce the mortality from chronic heart failure by 50%. Unfortunately, such benefits are not commonly observed in normal patient populations due to non-compliance to a daily regime of weight monitoring, even when internet-connected weighing scales are used to remove any necessity for patients to record or report the information collected.
Brijker et al. (Clinical Physiology, volume 20, issue 1, pages 56 to 61, January 2000) demonstrated that limb volume is a much more sensitive measurement for the purposes of heart failure monitoring than weight gain. Weight measurement has a number of known fluctuations, such as level of hydration and how recently ones bowel has been evacuated that can interfere with the desired signal indicative of excess water retention. The study of Brijker et al. demonstrated that patients showed a change of approximately 5.9% in weight between hospital admission and discharge, compared to a change of 13.1% in leg volume and a change of 7.1% in leg circumference. In addition, the coefficient of variation between weight and leg volume measurement was only r=0.37, suggesting that the two methods are somewhat independent, therefore leg volume monitoring may add significantly to the assessment of edema compared to the measurement of weight alone.
Unfortunately, the clinical gold-standard for limb volume measurement is a water displacement technique that suffers significant inter-operator variation and is cumbersome and error-prone to administer. Essentially, the limb of interest is immersed into a container of water to various depths, and the water displaced from the container is captured and recorded.
Because patients are capable of surviving for many years following the onset of chronic heart failure, the problem of finding a solution to the routine monitoring of changes in limb volume for the casually compliant patient in a home setting is an area of activity.
Therefore, a multitude of devices have been devised to measure changes in limb volume, typically in the form of various “smart” socks or similar, as is described in U.S. Pat. No. 8,827,930B2. The fundamental issue with such approaches is that patient non-compliance is a significant factor in effective long-term monitoring.
An alternative, non-invasive approach is to apply “machine vision” concepts, wherein the limb in question is measured using a 3D imaging system. Hayn et al. applied the Microsoft Kinect depth-measuring camera system to the measurement of leg volume for the detection of edema (AT513091B1). The approach taken by Hayn et al. was to identify a specific set of reference points, lines, curves or planes that had been shown to correlate with limb volume. Unfortunately, this approach provided only modest correlation with weight, and also requires that the patient is oriented in a particular way with respect to the imaging system in order to capture the reference points/lines/curves/planes. This is clearly susceptible to the same compliance issues noted above.
Other approaches to 3D imaging of limbs have been taken, typically in the hospital setting which involves the use of a 3D imaging system for routine measurement of limb volume. Unfortunately, such systems typically require the patient to be rotated with respect to the imaging system, and to have limbs not of interest to be covered so as not to interfere with the observation of the limb of interest. This is clearly not practical for home use.
An alternative approach is described in EP0760622A1, which describes a particularly inexpensive 3D scanner, in which a body part to be digitized is provided with an elastic cover that carries marks to be evaluated by photogrammetry. Overlapping images are taken using one or more roughly positioned cameras, and a 3D model of the body part is generated from the combination of these 2-dimensional (2D) images. Unfortunately, such a system is impractical for home-use by the casually compliant patient as they cannot be expected to diligently wear special clothing.
Stocking-based systems have been the subject of significant academic research, for example Hirai et al. in “Improvement of a three-dimensional measurement system for the evaluation of foot edema” (Skin Research and Technology, volume 18, issue 1, pages 120 to 124) and Pratsch et al. in “Perceived swelling, clinical swelling and manifest lymphoedema—difficulties with the quantifying of leg swellings” (Phlebologie, volume 41, page 5 to 11, 2012). However, the issue with usability remains significant.
Other methods and devices relating to image analysis are described in U.S. Pat. No. 8,908,928B1, US2011295112A1 and US2015216477A1.
Accordingly, an aim of the present invention is to provide a non-invasive, opportunistic method and apparatus for monitoring the peripheral edema associated with a range of pathologies including heart failure in animals or humans, and reporting the results thereof.